This invention relates to multimode optical fiber having controlled mode coupling and/or selective mode attenuation, and to methods of making the fiber.
Multimode optical fibers may have either a step index refractive profile or a graded-index refractive profile. In fibers with either type of index profile, the intermodal dispersion of an optical pulse propagating in the fiber typically contributes significantly to the spreading of the pulse, thereby limiting the bandwidth of the fiber for information transfer. Although graded-index structures are designed to minimize intermodal dispersion, practical limitations on industrially applicable manufacturing techniques often result in fibers with significant deviations from an optical index profile, and thus, fiber bandwidths well below that obtainable in an optimal fiber. This problem is especially pronounced in regard to plastic optical fiber.
It is well known that optical scattering in a multimode fiber not only increases optical attenuation, but can also decrease the intermodal dispersion of the fiber, either by producing coupling between the various propagating modes of the fiber, or by preferentially attenuating the modes primarily responsible for dispersion. However, because it is difficult to introduce mode coupling and differential mode attenuation in a controlled fashion, these remedies have not received significant commercial exploitation as methods for increasing the bandwidth of multimode optical fibers and communications systems using such fibers. In U.S. Pat. No. 6,304,705B1 of Kalish et al., issued Oct. 16, 2001, there is disclosed an optical energy transmission system having improved mode coupling. In that system, an optical fiber includes a plurality of particles formed in one or more coating layers surrounding the cladding layer of the fiber, and one or more buffer layers, but not in the core or cladding of the fiber. The particles cause perturbations, i.e., microbending, in the optical fiber which, in turn, enhance mode coupling, which produces a reduction in modal dispersion which, in turn, improves the bandwidth characteristic of fiber. The particles or bubbles in the outer coatings are not encountered by the light propagating through the core and cladding, but create stresses that alter the optical characteristics of the fiber. However, some instability may occur because of relaxation of the stresses within, for example, the buffer layer.
The present invention pertains to multimode optical fibers with novel structures, and their use in optical communications systems. In one embodiment, the invention comprises a multimode optical fiber with controllable mode coupling, which may be optimized to produce a desired level of mode coupling with minimal added optical attenuation. In another embodiment of the invention, the multimode fibers thereof controllably increase optical attenuation of certain modes with respect to that of other modes. The modes attenuated are those which must heavily contribute to dispersion. The fabrication processes of the invention produce these novel structures and are readily compatible with existing manufacturing techniques for either graded-index or step-index multimode fiber. They may also be optimized for a variety of optical fiber material systems, including silica glasses, as well as polymeric materials composed of either hydrocarbon polymers (e.g., polymethylmethacrylate or polycarbonate) or flourinated polymers such as poly(perfluoro-butenyl vinyl ether).
According to the invention, a communication system may be implemented by using an optical source and an optical detector, connected by a multimode optical fiber which contains particles within the core and/or the cladding layer whose refractive index is different from that of the surrounding fiber material. The refractive index structure introduced by the included particles may be imposed on a background refractive index structure of either a step-index or graded-index type of fiber. As optical pulses propagate through the fiber, they encounter the included particles and undergo scattering (and optionally absorption) thereby introducing mode coupling in the fiber. The size distribution, location, and refractive index of the particles may be chosen to produce a desired level of mode coupling, while suitably adjusting other parameters (e.g. by minimizing optical attenuation or by increasing differential attenuation of certain high-dispersion propagating modes with respect to other low-dispersion propagating modes). With the particles being contained in the core and cladding itself rather than in exterior coatings, greater mode coupling and greater stability results, inasmuch as there is no relaxation of the particle effect. The particles themselves operate directly on the light energy and not through the mechanism of microbending.
In a first embodiment of the invention, the multimode fiber is comprised of a polymer material with imbedded particles in the core thereof which are comprised, for example, of spherical beads composed of either silica glass, another polymer, or an electrically conductive material, or bubbles to control mode coupling with a minimum of optical attenuation. Such a fiber can be manufactured by dispersing the spherical scattering particles in a glassy polymer, and then using either a preform process or a co-extrusion process to produce from the glassy polymer step-index or graded-index optical fiber with imbedded particles.
In a second embodiment of the invention, fabricated by means of the co-extrusion process, scattering particles are added to the polymer material which forms the cladding layer of the fiber, thereby controlling the differential attenuation between the higher and lower order modes propagating within the fiber. Optionally, dispersal of the scattering spheres in the polymer material may be enhanced by appropriate chemical functionalization of the surfaces of the scattering particles. For example, silica spheres might be prepared with an organic surface treatment that enhances the solubility of the silica spheres in either an appropriate solvent or in the glassy polymer that forms the optical fiber. Such functionalized spheres can then be readily dispersed into a solvent and/or polymer solution to prevent the spheres from aggregating upon being mixed into the polymer.
In another embodiment of the invention, the multimode optical fiber is comprised of a silica glass, with imbedded particles comprised of an inorganic material. Optionally, such a fiber could be manufactured by dispersing the inorganic scatterers in a solution that is cast into a solid silica-based preform body using a sol-gel process. The resulting preform may then be drawn into an optical fiber using well-known methods. Optionally, chemical functionalization of the particle surfaces may be employed to promote dispersion.
The structures and techniques of the invention overcome problems inherent in prior art. For example, prior art structures and fabrication methods for multimode communications fiber often display reduced manufacturing yields associated with intermodal dispersion, resulting in higher manufacturing costs. Also, since the intermodal dispersion observed under conditions of restricted launch typically varies with the modal power distribution of pulses launched into the fiber, prior art communication systems often require relatively stringent conditions to benefit from restricted launch techniques. By introducing optical fibers with reduced intramodal dispersion and/or increased attenuation of certain optical modes, the invention offers the possibility for communications systems using simplified forms of restricted launch. The invention by allowing greater control over the mode coupling and differential mode attenuation of the multimode fiber in an optical communication system, enhances the ability of silica and polymer multimode fibers to serve as optical communication media.